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Cats are delicate drinkers, physics shows

How do cats lap? Cutta Cutta is about to take a drink.
[Micaela Pilotto, Roman Stocker and Pedro Reis]

Cat-and dog-owners already know that their beloved animals are completely different from each other, but scientists now have more evidence that relates to how our furry friends lap up liquids. While dogs plunk their tongues in water to scoop it up and fill their cheeks, cats use two physical forces—gravity and inertia—to delicately suck up liquids with just the tip of their tongue.

This research appears in the 11 November issue of the journal Science Express.

Much like elephant trunks and octopus arms, cat's tongues are remarkably agile given the lack of skeletal support. Therefore, the findings may provide new inspiration for soft robotics technology.

How do cats lap? Cutta Cutta is about to take a drink.
[Micaela Pilotto, Roman Stocker and Pedro Reis]

In the study, Pedro Reis and colleagues use high-speed imaging to capture the balance of forces that underpin cat lapping and the mechanics of water being lapped. It turns out that cats curve their tongue backward so that the top surface lightly touches the liquid.

When a cat raises its tongue rapidly, water is drawn up into a liquid column that grows by inertia. The cat then closes its jaw to capture the liquid before gravity breaks up this column. To get a better idea of the mechanism behind lapping, the team performed physical experiments in which a glass disk placed on a water surface (mimicking a cat's tongue as it leaves the water) was pulled upwards.

High-speed imaging showed water being stretched upward to form a column, just like in lapping cats. Experiments with different lapping speeds allowed the researchers to quantify the competing roles of gravity and inertia in setting the optimal lapping frequency.

Combined, these experiments revealed a few surprises about cat lapping—in particular the discovery that lapping is actually very different from the way dogs lap. Another surprise was the rapid speed at which at cat's tongue moves - nearly one meter per second.

The authors also discovered that they could use the balance of inertia and gravity to predict how fast a cat is lapping. They tested this hypothesis by measuring the lapping frequency for eight species of felines, from videos acquired at the Zoo New England or on YouTube. In agreement with their formula, the larger a cat was, the slower it lapped.